1. Field of the Invention
This invention relates to a driving circuit for a matrix type display device, and more particularly to a driving circuit for a matrix type liquid crystal display device.
2. Description of the Prior Art
Because of rapid advances in design and manufacturing technology in recent years, matrix type liquid crystal display devices are beginning to have a display quality which can match that of cathode-ray tubes. With its excellent features such as thin and light weight construction and low power consumption, a matrix liquid crystal display device is finding application in a variety of fields such as a display unit for a television receiver, a visual display unit for a personal computer and other information apparatus, and so on.
FIG. 3 shows one example of a conventional matrix type liquid crystal display device. The liquid crystal display device shown in FIG. 3 comprises a TFT liquid crystal display panel 100, a gate driver 200, and a source driver 300. In the display panel 100, picture elements 103 are arranged in a matrix of n rows and m columns, and thin-film transistors (TFTs) 104 are used as switching elements for driving the picture elements 103. Other transistors such as MOS transistors may be used as the switching elements. An array of the picture elements 103 arranged in a horizontal direction forms one horizontal scanning line. The TFTs 104 are respectively disposed adjacent to each picture elements 103. The drain of each TFT 104 is connected to an electrode of the corresponding picture elements 103. A counter electrode 105 is disposed as the other electrode which is common to all the picture elements 103. On the TFT liquid crystal display panel 100 are disposed an n number of scanning electrodes 101 parallel to one another. To the jth scanning electrode 101, the gates of the TFTs 104 corresponding to the picture elements 103 of the jth horizontal scanning line are connected. An m number of signal electrodes 102 are disposed parallel to one another and intersecting at right angles with the scanning electrodes 101. To the ith signal electrode 102, the sources of the TFTs 104 on the ith column are connected.
The TFT liquid crystal display panel 100 is driven by the gate driver 200 (vertical scanning means) and the source driver 300 (video signal output means). The gate driver 200 and the source driver 300 are connected to the scanning electrodes 101 and the signal electrodes 102, respectively. A video signal is input to the source driver 300. Control signals such as scanning pulses to the gate driver 200, and sampling clock pulses to the source driver 300 are fed from a control circuit not shown.
The display operation of the matrix type liquid crystal display device shown in FIG. 3 will be described with reference to FIG. 4. As shown in FIG. 4, the gate driver 200 applies a gate-on signal sequentially to the scanning electrodes 101 on the display panel 100. That is, the gate driver 200 scans the horizontal scanning lines in a predetermined sequence. A time TH is allotted to the scanning of one horizontal scanning line. When the jth scanning line is scanned, the TFT 104 connected to the jth scanning electrode 101 is turned on. The source driver 300 samples the input video signal at a predetermined frequency, and feeds the sampled video signal to the signal electrode 102 in synchronism with the gate-on signal output from the gate driver 200. Thus, the video signal is written in the picture element 103 through the activated TFT 104. The signal written in the picture element 103 is retained for a time T.sub.V till the next signal is written therein. In FIG. 4, T.sub.W indicates a period of time during which a video signal is written in a picture element.
The writing operation of the above-mentioned conventional driving circuit will be described in more detail referring to FIG. 5 which shows one of the output stages of the source driver 300. The output stage shown in FIG. 5 corresponds to one signal electrode 102.
The video signal is stored in a sampling capacitor C.sub.SMP when a sample pulse is input. Before writing the video signal into the corresponding picture element 103, a discharge signal DIS is turned HIGH, as shown in FIG. 6, to erase the previously written signal from the signal electrode 102. This causes the signal electrode 102 to be discharged through a transistor 303, resulting in that the potential of the signal electrode 102 drops to the ground level. Then, a transfer signal TRF is turned HIGH to transfer the video signal stored in the sampling capacitor C.sub.SMP to a hold capacitor C.sub.H, while the video signal is output through an output circuit including a differential amplifier 301, an output transistor 302 and transistors 304 and 305, to the signal electrode 102 connected to an output line 306. The transistor 305 functions to supply a bias current. At the same time when the transfer signal TRF is turned HIGH, the gate driver 200 turns on the TFTs 104 connected the applicable scanning electrode 101, and the video signal on the signal electrode 102 is written into the picture elements 103 connected to the energized TFT 104.
In the above-mentioned conventional driving circuit, the source driver 300 is not provided with a means for lowering the voltage of the signal electrode 102 when the voltage level of an input signal V.sub.IN is lower than the voltage of the signal electrode 102. Therefore, it is necessary to discharge the signal electrode 102 by means of the discharge signal DIS prior to the writing. As is apparent from FIG. 6, the presence of the discharge signal DIS reduces the period of time for writing the video signal into the picture element 103. This causes the charge characteristic of the picture element 103 to be impaired, thereby hindering the improvement of the contrast of the matrix type liquid crystal display device.
Also, since all the signal electrodes 102 are discharged at the same time by the DIS signal, a large discharge current flows into the source driver 300. Furthermore, since the discharge of the signal electrodes is performed at every scanning of a horizontal scanning line, the source driver 300 consumes a large amount of power.